KINEMATICS OF FAULTS IN THE HERRICK STOCK, NORTHERN IDAHO, AND IMPLICATIONS FOR THE MOVEMENT HISTORY OF THE LEWIS AND CLARK FAULT ZONE

Detailed kinematic analysis of minor structures within the Herrick stock, near St. Maries Idaho, provides new evidence for the timing and style of deformation along the Lewis and Clark line. The Lewis and Clark line is a continental-scale fault zone that links Eocene extension in Montana and Idaho with that in Washington. The Herrick stock is a Tertiary pluton that has been offset ~25km in a right-lateral sense by the St. Joe fault, which in this area displays southeast trending (118° - 128°) sub-vertical breccia zones with horizontal slickenfibers. The Herrick stock contains two sets of geologic structures. The first are thin (<4cm) ductile thrust faults that strike to the northeast (55° - 75°) and record compression along a northwest-southeast direction, with a sub-vertical minimum stress direction. The second are steeply dipping conjugate fracture sets (2 or more) that are lined with chlorite and thin microbreccia. The northeast striking fractures (10° - 40°) contain generally down-dip slickenlines and record extension along a northwest-southeast direction, with a sub-vertical maximum stress direction. The thrust faults can be explained by deformation of the pluton during Eocene right-lateral shear along the St. Joe fault. In contrast, many of the steep extensional fractures are not in an orientation that is compatible with a right-lateral sense of shear. They may record a distinctly younger episode of deformation, or movement along the north-trending White Rock detachment fault, which is exposed south of the study area.